JPH07124716A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent the longitudinal crack on the surface of a cast slab as the largest obstructing factor at the time of executing direct rolling and to provide the good cast slab without impairing the productivity and the profitability. CONSTITUTION:Electromagnetic stirring in a mold is executed before charging powder at the time of starting the casting. The solidified temp. of the powder used at the initial stage of the casting is made to be <=1000 deg.C. The electromagnetic stirring in the mold is executed in the condition of obtaining 0.4-1.2m/sec flow rate before charging the powder at the time of starting the casting and the powder having <=1000 deg.C solidifying temp. is used at the initial stage of the casting and the powder having >=1150 deg.C solidifying temp. is used at the middle stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for preventing surface vertical cracking of a slab.

[0002]

2. Description of the Related Art Conventionally, various defects such as vertical cracks and lateral cracks are liable to occur in a slab produced by a continuous casting method, and in particular, a so-called sub-content containing 0.1 to 0.18% carbon is contained. It is well known that it occurs frequently in the steel types in the peritectic region.

[0003] When a steel type which is likely to cause vertical cracks on the surface of a slab in the sub-peritectic region is directly fed and rolled, a defect called a "health flaw" occurs in a hot rolled coil or a thick plate product. Removal measures or protective measures such as not allocating to direct rolling have been taken to remove or originally designate as cold piece inspection.

[0004] Many studies have been made on the mechanism of preventing such vertical cracks and a method for preventing them, and the effects have been exhibited as such.

It is clear from these studies that vertical cracks are caused by uneven solidification in the initial solidification process in the vicinity of the meniscus of the mold, and are caused by the concentration of strain in the cooling process in the solidification delay part. It has become to.

Therefore, as a preventive measure, it is firstly necessary to promote the homogenization of the initial solidified shell which triggers vertical cracking, and secondly, it is important to try to alleviate the strain concentrated there. That's a good thing.

From this point of view, various measures are first taken for the uniform formation of the initially solidified shell. For example,
A method of controlling the viscosity and melting point of the powder to ensure uniform inflow and making the initial solidified shell uniform, or a method of applying electromagnetic stirring in the mold to improve the uniformity of the initial solidified shell are used. Has been.

On the other hand, the generated solidified shell is distorted during cooling due to heat shrinkage, external restraining force, etc., and strain concentration occurs in the solidified delay portion, resulting in surface cracking. In order to control the cooling rate of the cast slab to a certain value or less, slow cooling is performed by controlling the spray density of the cooling water in the secondary cooling zone of the continuous casting machine, or even if distortion occurs, it is concentrated. In order to avoid this, a method of avoiding it by controlling the physical properties of the powder used as a mold lubricant is adopted.

Further, if a powder having a high crystal crystallization temperature (abbreviated as BP) is used for the purpose of further slow cooling even in the mold, the heat conduction is lowered, and as a result, slow cooling is promoted. Therefore, the occurrence of the strain is reduced, and it is effective in preventing the surface cracks and flaws of the cast slab.
It is disclosed in Japanese Patent Laid-Open No. 961 or Japanese Patent Laid-Open No. 4-224063.

On the other hand, by applying an appropriate flow velocity in the vicinity of the mold meniscus by electromagnetic stirring, a uniform dynamic pressure and heat flow velocity are imparted without causing re-melting of the initial solidified shell, and the contact state between the solidified shell and the mold is improved. A method of maintaining and preventing vertical cracking is disclosed in Japanese Patent Application Laid-Open No. 1-228645.

However, it has been found as a result of a casting experiment that even if the methods described in these documents are applied, the rate of occurrence of vertical cracks in the slab does not improve at all, and there is no effect.

[0012]

DISCLOSURE OF THE INVENTION The inventors of the present invention conducted a casting experiment under a steel type / casting condition in which the vertical crack is likely to occur, and as a result of various investigations on the cause, the vertical crack occurrence rate in the casting direction of the cast slab was found. As shown in FIG. 1, it was clarified that the number of transitions was the highest in the initial stage of casting and was extremely stable in the middle stage of casting.

Further, the inventors of the present invention analyzed the initially solidified shell in the initial stage of casting of a slab in which vertical cracking occurred, collected inflowing powder, and investigated and analyzed the characteristics thereof.
It was found that the thickness of the powder flowing in the initial stage of casting was extremely non-uniform, resulting in non-uniform solidification shell.

[0014]

The present invention provides an excellent means for preventing vertical cracking of a slab produced by continuous casting based on the above findings. Its characteristic features are as follows.

(1) When producing a cast slab by the continuous casting method, it is necessary to stir molten steel at a flow velocity of 0.4 to 1.2 m / sec by electromagnetic stirring before introducing powder before starting casting. Characteristic continuous casting method.

(2) When producing a slab by the continuous casting method, the solidification temperature of the powder used at the initial stage of casting is 1000
℃ or less, then solidification temperature is 1150 during steady casting
A continuous casting method characterized by using powder having a temperature of ℃ or higher.

(3) When producing a slab by the continuous casting method, the molten steel is stirred by electromagnetic stirring at a flow velocity of 0.4 to 1.2 m / sec prior to the introduction of powder before the start of casting, and A continuous casting method characterized in that the powder used at the initial stage of casting has a solidification temperature of 1000 ° C. or lower, and then the powder having a solidification temperature of 1150 ° C. or higher is used during steady casting. Is.

[0018]

In order to solve the above-mentioned problems, the present inventors conducted experiments in which the physical properties of the powder used in the initial stage of casting and the electromagnetic stirring conditions in the initial stage of casting were variously changed, and the properties of the initial solidified layer of the slab and the mold / The powder that flowed in between the solidified shells was collected and the actual condition was investigated and analyzed.

Here, the steel type used in the casting experiment is C:
0.10 to 0.18%, Mn: 0.4 to 1.6%, S
i: 0.08 to 0.50%, P: 0.002 to 0.02
It is a general medium carbon steel with 5% and S: 0.001 to 0.020%.

First, the mold /
When the thickness distribution in the width direction of the powder collected from between the solidified shells was investigated, it was found that there was a large difference in the distribution in the early and middle stages of casting, as shown in FIGS.

That is, it was found that, as shown in FIG. 2, that in the initial stage of casting was extremely non-uniform, whereas in the middle stage of FIG. 3, it was extremely uniform.

Further, it was also found that even powders in the initial stage of casting have large nonuniformity and uniform ones.

On the other hand, the state of occurrence of vertical cracks on the surface of the slab was examined, and a large vertical crack with an opening at the center of the width was found to occur in the slab where the thickness of the powder was non-uniform at the beginning of casting. On the other hand, it was also found that vertical cracks did not occur when they were uniform.

The present inventors have conducted further detailed analysis to
The condition that the powder that flowed in between the solidified shells was uniform was that, firstly, the physical properties of the powder (so-called front powder) used in the initial stage of casting had a viscosity of 1300 ° C. of 1
It was found to be ~ 6 poise, and the melting point is in the range of 1000 to 1200 ° C, which is not different from the powder that is usually used, but it was found that the solidification temperature is limited to 1000 ° C or less.

However, it was also found that when the powder is continuously used until the middle stage of casting, the powder thickness gradually becomes nonuniform and vertical cracks on the surface of the slab also increase.

Therefore, it becomes necessary to use powders having different characteristics in the middle stage of casting.

As a result of further analysis on this point, it was also found that the use of a powder having a solidification temperature of 1150 ° C. or higher can prevent this adverse effect.

The second feature is due to the difference in the start timing of electromagnetic stirring in the mold. Before the powder is added immediately before the start of casting, the stirring is started at a molten steel flow rate in the range of 0.4 to 1.2 m / sec. In the case where the powder is uniform in thickness, on the other hand, when stirring is started after the start of casting, when the low solidification temperature powder is applied,
It has been found that the variation in the thickness fluctuates extremely greatly and that vertical cracks in the slab frequently occur.

When the molten steel flow velocity is less than 0.4 m / sec, the effect of improving the uniformity of the powder is not recognized,
On the other hand, it has been found that when a flow velocity exceeding 1.2 m / sec is given, powder is entrained in the slab and these powders lead to product defects during hot rolling.

Further, electromagnetic stirring in the mold was applied before the powder was added just before the start of casting, and the molten steel flow rate was set to 0.4.
It has been found that when the powder having a solidification temperature of 1000 ° C. or lower is used in the initial stage of casting, the dispersion of the powder thickness becomes the smallest when stirring is performed in the range of up to 1.2 m / sec.

The present invention is based on the above findings.

[0032]

EXAMPLES Examples of the present invention and comparative examples will be specifically described below.

Continuous cast slabs produced under the conditions shown below using the steels having the components shown in Table 1 were subjected to the powder physical properties shown in Table 2 and the electromagnetic stirring conditions in the mold shown in Table 3, A continuous casting test was performed to observe the presence or absence of vertical cracks on the surface of the slab. The results are shown in Table 4 together with the casting conditions.

As can be seen from the above, the continuous cast slab produced according to the present invention, in particular, the vertical cracks generated at the initial stage of casting,
It can be prevented very efficiently.

The method for producing the continuously cast slab is as follows. Continuous casting slab size; thickness 200/300 mm x width 18
00 / 2000mm Casting speed; 1.0m / min to 1.8m / min Model of continuous casting machine; Single arc type and vertical / bending type continuous casting machine (vertical part of vertical / bending type continuous casting machine is 2.5m) Specification of electromagnetic stirrer in mold

[0036]

[Table 1]

In FIG. 5, 0.6 m before the start of casting.
Inflow powder sampled between the mold / solidified shell sampled during casting when the electromagnetic stirring in the mold under the condition of ensuring a flow rate of / sec is applied and the solidification temperature of the powder applied in the initial stage of casting is 980 ° C. The thickness profile in the width direction is arranged in the same manner as in FIGS. 2 and 3.

As is clear from this result, an extremely uniform inflow is obtained even in the initial stage of casting when the nonuniform inflow originally occurs.

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

[0044]

[Table 7]

[0045]

[Table 8]

[0046]

[Table 9]

[0047]

INDUSTRIAL APPLICABILITY As described in the above-mentioned actions and examples, the present invention is particularly suitable for producing a slab by continuously casting a so-called medium carbon steel having a carbon content of 0.1 to 0.18%. Vertical cracks that occur at the initial stage of casting can be reliably suppressed, and good cast pieces can be manufactured, which enables speeding up and maintenance-free, and the resulting economic effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram for explaining that the occurrence state of vertical cracks in the casting direction of a continuously cast slab produced by the continuous casting method changes for each pot of continuous casting.

FIG. 2 is a diagram illustrating a widthwise distribution of inflow powder collected from between a mold and a solidified shell at the initial stage of general casting.

FIG. 3 is a diagram for explaining a widthwise distribution of inflow powder collected from between the mold and the solidified shell in the middle of casting during general casting.

FIG. 4 is an explanatory view showing the outline of a mold used in the embodiment of the present invention and an electromagnetic stirring device provided therein.

FIG. 5 shows the uniformity in the width direction of the inflow powder between the mold and the solidification shell when the electromagnetic stirring in the mold is applied before the initial casting powder is added and the solidification temperature is less than 1000 ° C. Explanatory drawing which shows that it improved.

[Explanation of symbols]

 1 Mold long side 2 Mold short side 3 Mold width varying device 4 Mold traverser 5 Mold water cooling box 6 Connector box 7 Electromagnetic stirrer 8 Vibration frame 9 Core 10 Coil 11 Slab

Claims (3)

[Claims] 1. When producing a slab by a continuous casting method,
A continuous casting method characterized in that molten steel is stirred by electromagnetic stirring at a flow rate of 0.4 to 1.2 m / sec prior to the introduction of powder before the start of casting. 2. When producing a slab by a continuous casting method,
A continuous casting method, characterized in that the solidification temperature of the powder used in the initial stage of casting is 1000 ° C. or lower, and then the powder having a solidification temperature of 1150 ° C. or higher is used during steady casting. 3. When producing a slab by a continuous casting method,
Before the start of casting, the molten steel is stirred by electromagnetic stirring at a flow rate in the range of 0.4 to 1.2 m / sec prior to the introduction of powder, and the solidification temperature of the powder used at the initial stage of casting is set to 1000 ° C or less, and then steady. A continuous casting method characterized in that powder having a solidification temperature of 1150 ° C. or higher is used during casting.